The present invention relates to a plasma processing device for subjecting an object such as a semiconductor wafer to a predetermined process such as film formation.
The invention also relates to a method of subjecting an object such as a semiconductor wafer to a predetermined process such as film formation by using a plasma.
With recent development in enhancing integration density and miniaturization of semiconductor products, plasma processing devices have been used in some cases in order to perform processes such as film formation, etching and ashing in steps of manufacturing the semiconductor products. In particular, a microwave plasma device tends to be used, since it can create a stable plasma even in a high vacuum state with a relatively low pressure of about 0.1 to 10 mTorr. In the microwave plasma processing device, a high-density plasma is created by combining microwaves and a magnetic field generated from a ring-shaped coil.
For example, there is known a conventional microwave plasma device wherein a plasma generating chamber having magnetic field generating means is provided with a microwave introducing port and an electron cyclotron resonance space is produced. Ions are extracted from the plasma generating chamber, and a process gas in a reaction chamber is activated by the plasma, thus performing various processes such as film formation.
FIG. 1 is a schematic diagram showing the structure of such a conventional plasma processing device. In the figure, a process chamber 11 is formed of, e.g. aluminum in a cylindrical shape. A table 12 for mounting of a semiconductor wafer W as an object to be processed is provided within the process chamber 11. An upper part of the process chamber 11 is narrowed in a stepwise fashion and a plasma chamber 13 is formed in the upper part. A reaction chamber 14 is formed below the plasma chamber 13.
A ceiling cover 15 of, e.g. quartz for sealing a ceiling portion of the process chamber 11 is airtightly provided on the upper part of the plasma chamber 13. The ceiling cover 15 constitutes a microwave introducing window 16. A conical taper waveguide 17 is connected in a member to face the microwave introducing window 16. A rectangular waveguide 18 is connected to a top portion of the taper waveguide 17. A microwave generator 19 for generating microwaves of, e.g. 2.45 GHz is provided on the rectangular waveguide 18. Microwaves generated by the microwave generator 19 are introduced into the plasma chamber 13 through the microwave introducing window 16 via the rectangular waveguide 18 and taper waveguide 17.
Ring-shaped main coils 20 and sub-coils 21 are disposed outside the plasma chamber 13 of process chamber 11 and below the bottom of the chamber, respectively. Each coil 20, 21 generates a downward magnetic field and thereby a downward mirror field is produced within the entire process chamber 11. In this case, the downward magnetic field and microwaves are set to meet the condition for electron cyclotron resonance. Specifically, if the frequency of microwaves is 2.45 GHz, the magnitude of the magnetic field is set at about 875 gauss.
Thus, the plasma gas, e.g. argon gas, introduced into the plasma chamber 13 is made into a plasma by electron cyclotron resonance caused by synergetic effect of applied microwaves and magnetic field. The generated plasma activates a process gas, e.g. silane gas and oxygen used as film formation gas, supplied to a lower part of the plasma chamber 13. The activated process gas reacts and a reaction product deposits on the surface of the wafer as a thin film.
In the meantime, when the condition for electron cyclotron resonance is satisfied, the frequency of microwaves and the magnitude of magnetic field are definitively determined by setting the potential, mass, etc. of charged particles. However, if the frequency of microwaves is set at 2.45 GHz, as mentioned above, the main coils 20 and the sub coils 21 for obtaining the corresponding field intensity of 875 gauss become very large in size. For example, the weight of the main coil 20 becomes 100 Kg or more. Consequently, the cost for the plasma processing device increases, the maintenance work for the plasma processing device is time-consuming and the space for installation of the apparatus cannot be decreased.
In particular, in the case of a plasma processing device for processing 12-inch wafers, the diameter of the table 12 further increases, as compared to the apparatus for processing 8-inch wafers. Consequently, the size of the coil further increases, and the weight thereof becomes, for example, about 200 Kg. Under the circumstances, with an increase in diameter of the wafer, there is a demand for reducing the size of the coil.
In addition, when microwaves are supplied into the process chamber 11, an impedance variation of the plasma will inevitably occur due to a variation, etc. in density of the generated plasma. Thus, all microwave power output from the microwave generator 19 is not supplied into the process chamber 11, and some reflection power will occur due to mismatching of impedance.
In this case, an effective power contributing to plasma generation is equal to a difference between the output power and reflection power. In the prior art, however, no measure is conducted to control reflection power, and only output power is controlled. Thus, different powers may be supplied to wafers, depending on the impedance state of plasma. Consequently, reproducibility of process may deteriorate.
Furthermore, such reflection power is wasted since it does not contribute to plasma generation. From the standpoint of power consumption, the presence of reflection power is not desirable.